Is this the biggest wave ever surfed?

A Hawaiian surfer may have beaten his own world record for riding the largest wave ever surfed - a monster 30-metre wall of water off the coast of Portugal.

If the claims are verified, big wave surfer Garrett McNamara will go into the Guinness World Records with his incredible ride on a wave estimated to be more than six metres higher than his previous record, set at the same spot at Nazare in Portugal last year.

It means the world to me. Today was an awesome day and so fun to be out there

Photographs and video taken on January 28 show the 45-year-old as a small speck on a towering wave as he plunges down its face. He rides numerous sets, at one stage wiping out on the face of a wave and diving into the churning water.

Surfing the world's biggest wave

US surfer Garrett McNamara may have beaten his own world record riding what is thought to be a 30.5 metre wave at Nazare, Portugal. Photo: AP

McNamara was towed by a jetski onto the largest wave, which formed over an undersea canyon known as one of the biggest wave-generators on the planet.

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Last year, Guinness World Records recognised McNamara for catching a 78-foot (23.77-metre) wave at the same location, but McNamara's support team believe this week's wave was even larger than that record-breaking ride.

The town's surfing company, Nazare Qualifica, said it was trying to find out exactly how massive the wave had been.

"Authorities from the company have asked two experts from the US to certify the size of the wave," the Lusa news agency reported.

Miguel Sousinha, head of Nazare Qualifica, said: "McNamara's people think that the wave he surfed today is even bigger than that of 2011."

He said the experts usually worked with the Billabong XXL surfing contest organisers.

Billabong XXL wrote on its Facebook page afterwards that Nazare had been hit by a massive north-west swell. "Perfect conditions! McNamara was charging!"

McNamara tweeted his thanks for all the support.

"It means the world to me. Today was an awesome day and so fun to be out there," he wrote.

McNamara is believed to have just flown into Portugal from his home in Haleiwa, on Oahu's North Shore, and it was his first day in the water when he caught the massive wave.

When he set the previous record last year, McNamara said the ride was a "fluke" and he originally didn't want to attempt the waves that day after wiping out numerous times on even bigger swells in the same spot.

"I was really beat-up that morning," he told the Associated Press. "This day, I did not want to get out of bed."

But he changed his mind after his friends encouraged him.

"Everything came together," McNamara said. "Everything felt right."

Last year he also was awarded $US15,000 for the ride at the Billabong XXL Global Big Wave Awards in California.

At the time, he described the unusual underground formations that caused such massive surf at Nazare.

"There is an underwater canyon 1000 feet [300 metres] deep that runs from the ocean right up to the cliffs. It's like a funnel. At its ocean end it's three miles [four kilometres] wide but narrows as it gets closer to the shore and when there is a big swell it acts like an amplifier," he told The Observer in 2011.

"The harbour where the jetskis are kept is about five minutes' ride away. I can see it from my hotel window. You go out and it can be almost flat as you leave and ride along the coast. You start seeing the waves after about half a mile when you pass some rocks and turn a point. Then you are in the break. It's unique. The waves break into cliffs 300 feet [91 metres] in height. You can't contemplate coming off because it would kill you."

In 2007, McNamara became the first man to ride a glacier wave on a surf board.

He spent three weeks camped out by the 120-metre Child's Glacier in Alaska waiting for the perfect wave.

He was towed onto a large wave that was generated when a huge block of ice broke off the glacier and fell into the water.